1.Technical Field:
The present invention relates to a control for use with a reverse osmotic. treatment system. More particularly, the invention relates to an apparatus for controlling the flow of a product fluid from a reverse osmotic treatment system using a fluid pressurizing means such as a centrifugal pump.
2. Prior Art:
A reverse osmotic treatment system is equipped with a reverse osmotic apparatus using semipermeable membranes. An impure fluid boosted by a pressurizing means of pump type is fed to the reverse osmotic apparatus, which separates the feed fluid into a relatively pure portion which has passed through the semipermeable membrane and is substantially made of the solvent, and a portion with a higher concentration of the solute.
The reverse osmotic treatment system is typically used in plants for desalinating salt waters such as brine, brackish and seawater. In order to produce desalinated water, the feed fluid must be pressurized to a value exceeding its osmotic pressure.
A system has recently been developed for desalinating a large volume of seawater with the reverse osmotic apparatus. In this system, the feed fluid is pressurized by a centrifugal pump in place of a reciprocating pump which is commonly employed in the small-scale system. This new type of reverse osmotic treatment system has a problem in connection with adjustment of the frow of the product solution (desalinated water when the feed solution is seawater, and a concentrated solution if the feed contains a chemical).
A typical configuration of the prior art technique for controlling the flow of product fluid from the reverse osmotic treatment using a centrifugal pump as a pressurizing means is shown in FIG. 1. The reverse osmotic treatment system shown is used to desalinate seawater. A centrifugal pump 1 driven by a motor 8 pressurizes the seawater and sends it to a reverse osmotic apparatus 4 composed of a plurality of reverse osmotic modules. Deslinated water coming out of the semipermeable membrane in each module is stored in a tank 6. A concentrated solution also comes out of the reverse osmotic apparatus but by a different channel and is squirted against a hydraulic turbine 7 of Pelton wheel type through a nozzle 7'. The power shaft of the turbine 7 is directly coupled to the rotor of the motor 8 and feeds back the rotary energy to the pump 1.
The control system shown in FIG. 1 includes an indicator 2a which indicates the pressure detected by a pressure sensor at the inlet 2' of the reverse osmotic apparatus (the indicator may be replaced by a pressure indication control which indicates a set value of pressure as well as the detected pressure), and a pressure regulating valve 2 provided on the primary or feed line between the pump 1 and the reverse osmotic apparatus which is controlled either manually or automatically by the control 2a to provide a feed pressure which is equal to the set value. The control system further includes a flow indicator 3a which indicates the flow detected by a flow sensor at point 3' on the feed line (this indicator may be replaced by a flow indication control with indicates a set value of flow as well as the detected flow), and a flow regulating valve 3 with a integral nozzle 7' which is disposed at the terminal end of the concentration or secondary line and which is controlled either manually or automatically by the control 3a to provide a flow of the concentrated solution that is equal to the set value, The line on which the desalinated water flows is provided with a flowmeter 5 for measuring the flow of the desalinated water.
In the prior art control, the reverse osmotic treatment system is operated in such a manner that the recovery percentage or the ratio of the flow of desalinated water to that of the feed solution is held constant, In some cases, it is desired to switch the system operation to such a mode that the desired flow of desalinated water is increased (or descreased) by a significant amount (e.g. tens of percent). However, with the prior art system, complicated valve operations are necessary to achieve the newly set value of the desired flow of desalinated water. For one thing, each of the valves 2 and 3 must be shifted bit by bit to avoid the hydraulic interference therebetween. For another, the final stage of valve adjustment is solely the responsibility of a skilled operator who makes the fine tuning by referring to the reading on the flowmeter 5. This is partly because it has been impossible to predict accurately the value of the feed pressure necessary to accomplish the desired flow of desalinated water although the flow of the feed solution can be determined by dividing the flow of desalinated water by a fixed recovery percentage. The time required to make full adjustment to a new operating mode is of the order of hours (typically 2 hours).
The pressure regulating valve 2 disposed on the feed line causes a pressure drop in the fluid, and this results in a corresponding decrease in the pressure of the fluid to be supplied to the reverse osmotic apparatus. As a result, a centrifugal pump having a relatively large capacity to compensate for the pressure drop caused by the valve becomes necessary, thus being accompanied by two disadvantages: lower efficiency of the production of desalinated water, and a relatively large power requirement.
A further disadvantage of the prior art control system is its high cost due to the use of a relatively large number of components (i.e. two valves, a pressure sensor, a pressure indication control, a flow sensor) and a flow indication control). Also, it is desired to make automatic checking of the individual components of the reverse osmotic treatment system for a decrease in their performance. More desirably, such a decrease should be evaluated quantitatively.